Designing voltage-controlled and digitally-controlled oscillators (VCO, DCO) of high spectral purity and low power consumption is quite challenging, especially for a GSM transmitter (TX), where the oscillator phase noise is required to be less than −162 dBc/Hz at 20 MHz offset frequency from the 915 MHz carrier. At the same time, the RF oscillator consumes a disproportionate amount of power of an RF frequency synthesizer and consumes more than 30% of the cellular RX power. Consequently, any power reduction of RF oscillators will greatly benefit the overall transceiver power efficiency and ultimately the battery lifetime.
In an oscillator, the phase noise depends on the quality factor (Q) of its LC tank, its oscillation voltage swing and its effective noise factor. The Q-factor at wireless cellular carrier frequencies is usually limited by the inductor due to physical constraints on the width and thickness of the metal and the substrate loss in bulk CMOS and does not change too much when migrating to advanced CMOS technologies.
On the other hand, the oscillation voltage swing is limited by the supply voltage VDD, which keeps on reducing in the advanced CMOS technology. In addition, increasing the oscillation voltage stops improving the phase noise when the gm-devices enter the triode region. Furthermore, the excess noise factor of the transistors is increased resulting in larger noise factor for the oscillator. Consequently, the phase noise and power efficiency of the traditional RF CMOS oscillator reduce by migrating to more advanced technologies. Prior art oscillators suffer from inadequate phase noise performance, clip-and-restore DCO due to use of two transformers (die area penalty) and large gate oxide swings (reliability issues); and die area penalties of utilizing an extra inductor as well as large VDD=2.5 V (noise-filtering oscillators).
There is thus a need to increase the power efficiency of an RF oscillator while meeting the strict phase noise requirements of the cellular standards with sufficient margin and abiding by the process technology reliability rules.